Mineralogy

The Mineral Kingdom

It has long been the custom to divide nature into three great departments, the animal, vegetable and mineral kingdoms. The mineral kingdom comprises the materials that make the crust of the earth and a part of this kingdom is dealt with in .the science of mineralogy. Whether or not any definite boundaries exist between the three kingdoms is a subject which remains to be investigated.

The different members of the animal and vegetable kingdoms are characterised by the development of special organs, or of certain peculiarities of structure, by means of which they pass through a series of changes known as life and growth. This latter phenomenon takes place by the absorption of various kinds of matter which then undergoes conversion by chemical processes into substances similar to those making the plant or animal. In this way the waste which accompanies life is replaced. The bones and shells of animals consist to a great extent of mineral matter. Plants are capable of deriving earthy substances from the soil in which they grow. But mineral matter which has thus been utilised by organisms passes, in the rigid interpretation of the term, beyond the pale of mineralogy, for it assumes a structure, governed by the nature and requirements of the animal or plant, that ‘it would not possess as an ordinary portion of the earth’s crust. For example, a pearl would be regarded as an organic substance and not a true mineral, although it consists of mineral matter. Again, coal, being a substance derived from the decomposition of vegetable matter, would not be rigidly classed with minerals.

Minerals

A most important characteristic of a mineral is the possession of a definite chemical composition. Some qualification of this statement is, however, necessary. Certain minerals form a closely related series in which there is a gradual replacement of one element by another, the two end-members of the series being connected by a number of transitional types of intermediate composition. In order to avoid the establishment of a great number of slightly differing mineral species, it is usual in such cases to consider the series as a whole, definite names being given to the end-members and possibly to certain intermediate types of historic or other interest. The variations of the chemical compositions of such series are not haphazard but are governed by certain rules.

The possession of a definite chemical composition does not suffice in all cases to fix the mineral species. It is found that two minerals with markedly different physical properties, such as colour, hardness, form, density and so on, have identical chemical compositions. In cases such as these, the two mineral species have their atoms arranged on different plans with the result that they have different physical properties. Under favourable conditions, the internal atomic structure of minerals finds expression in their external forms which are bounded by flat surfaces arranged in characteristic ways. Minerals with such external forms provide the beautiful objects known as crystals.

It follows from the requisite of a definite chemical composition and a definite atomic structure that minerals must be homogeneous, that is, each part, however small, must have the same chemical and physical properties.

Definition of a Mineral

A mineral is a substance having a definite chemical composition and atomic structure and formed by the inorganic processes of nature.

If we follow this definition rigidly, we are bound to consider the naturally occurring pure gases amongst the minerals. We should not include air, however, since it is a mixture of nitrogen and oxygen and is therefore not homogeneous. Again, water, snow and ice come within the definition since they are naturally occurring homogeneous inorganic substances of a definite chemical composition. The so-called mineral oils are mixtures of several hydrocarbons and therefore cannot be considered as mineral species.

What should be included within the rigid definition of a mineral is thus clear, but the term is often employed in a more extended sense, a usage which has been the cause of several celebrated law-suits. Thus, a miner considers a mineral to be anything of economic value that can be extracted from the earth. The national statistical summaries of mineral production include details of materials such as chalk, clay, coal, petroleum, and igneous rocks that do not come within the definition of a mineral. In this book it is proposed to discuss not only those substances which fulfill the term, but also a few materials whose origin may not always be free from organic causes or whose chemical composition may not be constant. Coal, mineral oils, limestones and some phosphate are examples of such substances.

Bodies in no way to be distinguished from actual minerals have at various times been artificially formed, either purposely in the laboratory or by accident in industrial processes; but although identical with true minerals of like chemical composition, they are the outcome of processes controlled by human agency, and consequently are not included among minerals. They have, nevertheless, a profound interest for the mineralogist inasmuch as they serve to a certain degree to elucidate the conditions under which the corresponding minerals have been formed.

Rocks

The popular usage of the term mineral includes, as we have already seen, certain substances which are more properly called rocks. A rock is a portion of the earth’s crust which has some individuality; it is the working unit of the field geologist and the distribution of the various kinds of rocks is shown upon geological maps. A rock has no distinctive shape of its own, it has no definite chemical composition and it is not homogeneous.

Examination shows that in most cases rocks consist of a mixture of various minerals. The heterogeneous rock can be taken to pieces and the several homogeneous minerals that compose it separated out. For example, consider the well-known rock granite. It can be seen by inspection of a hand-specimen of this rock that it is made up of three constituents-one white or pink and cleavable, which is the mineral orthoclase; another, clear glassy and with no cleavage, which is the mineral quartz; and a third, glistening, scaly and soft, which ‘is the mineral mica. Detailed chemical and physical investigation would show that the components, orthoclase, quartz and mica, fulfil the requisites of minerals. They are the mineral units which have been aggregated together to form the rock granite. These three constituents occur in varying proportions in different granites and even in different parts of the same granite mass. It sometimes happens that a rock, in the geological sense of an individual portion of the earth’s crust, may be composed of one mineral only. For example, a pure statuary marble consists of the single mineral calcite.